Methods for detecting a signal and evaluating a channel using a known preamble, are known in the art. Such methods are used in timed framed communication methods such as TDMA communication standard, CDMA communication standard, and the like.
Reference is made to FIG. 1 which is a schematic illustration of a timed framed TDMA sequence, referenced 1, known in the art. Frame sequence 1 includes a plurality of slots 2, 4, 6, 8, 10 and 11. Each of these slots includes bits of data, lasting a predetermined period of time.
In TDMA, several users may use a predetermined communication channel, each at predetermined intervals in time. In the present example, slot 2 is assigned to a first user, slot 4 is assigned to a second user, and so on.
According to common TDMA standards, such as IS-136, IS-54, RCR-27, the preamble (i.e. the first few bits of information) of each slot represents a known synchronization sequence, also known as a sync-word. Each of slots 2, 4, 6, 8 and 10 has a sync-word 12, 14, 16, 18 and 20, respectively.
Sync-words are used to determine what the channel looks like with regard to several aspects such as amplitude, phase, timing, frequency offset, reflections which produce echoes, represented by channel taps, distortions, interference and the like.
Multiple channel taps cause inter-symbol interference. For example, the value of a sample s.sub.i sampled at the output of the multi-path channel, is given by EQU s.sub.i =.SIGMA..sub.n=-L.sbsp.1.sup.L.sbsb.2 h.sub.n a.sub.i-n
wherein the a.sub.i are the transmitted samples (-.infin.&lt;i&lt;.infin.); and PA1 h.sub.n are the gains of the channel taps (-L.sub.1 .ltoreq.n.ltoreq.L.sub.2). L.sub.1 and L.sub.2 define the length of channel memory which, in turn, defines the number of neighboring symbols which affect each sample. PA1 calculating from the preceding slot synchronization sequence an estimated taps value for each of the synchronization sequence postulates; PA1 calculating from the preceding step synchronization sequence a log likelihood metric value C(y,h) for each of the synchronization sequence postulates; and PA1 selecting the synchronization word postulate having the best metric value. PA1 calculating a reliability value; and PA1 determining that the selected synchronization sequence postulate reliable. PA1 wherein the reliability value is calculated from comparing the selected synchronization sequence postulate with the at least one synchronization sequences postulate.
Conventional methods use the sync-word of each slot to determine the appropriate synchronization to this slot. Thus, the receiver of the first user, receiving slot 2, will analyze sync-word 12 (S1), determine the channel characteristics and proceed analyzing the rest of slot 2, accordingly.
It will be appreciated that a communication channel may develop in time, which affects the form of a received slot. Thus, an analysis, based on a sync-word at the beginning of a slot, might not be accurate for the last part of the slot.
Accordingly, conventional receivers execute dynamic tracking procedures, which track the channel development in time, while decoding the data contained therein. Temporary fading of a channel may cause considerable degradation in such channel tracking procedures.
Another method known in the art analyzes the channel of a received slot according to the sync-word of the slot and the sync-word of the next adjacent slot. For example, the receiver of the first user, receives slot 2 as well as the sync-word 14 of slot 4 and analyzes the channel from sync-word 12 (S1) and 14 (S2).
Analyzing the channel for slot 2 using sync-word 12 (S1), combined with forward in time channel tracking, is also called forward analysis. Analyzing the channel for slot 2 using sync-word 14 (S2), combined with backward in time channel tracking, is also called backwards analysis. According to prior art methods, this can be performed only when S1 and S2 are known, fixed sequences.
It will be appreciated that in some communication standards, such as TDMA IS-54, IS-136, RCR-27 and the like, the sync-word of the next slot, is not a fixed predetermined sequence, which make the above prior art methods inefficient.
These TDMA standard define a frame which includes six slots. TDMA standard also defines two modes of channel usage. A first mode is called half-rate, in which a user is assigned one slot (i.e. every sixth frame is reserved for his use). A second mode is called full-rate, in which a user is assigned two slots in a frame (i.e., every third slot).
Reference is now made to FIG. 2 which is a schematic illustration of a combined full-rate half-rate frame TDMA sequence, generally referenced 48. Frame 48 includes a plurality of data slots, 50, 52, 54, 56, 58, 60 and 62, each having a preamble 70, 72, 74, 76, 78, 80 and 82, respectively.
The first six slots define a frame, operative for a number of users, which will be repeated further in the frame. This frame is assigned to two half-rate users and two full-rate users.
Slots 54 and 60 are assigned to the first and second half-rate users, respectively. Slots 50 and 56 are assigned to the first full-rate user and slots 52 and 58 are assigned to the second full-rate user.
In the present example, preambles 70 and 76 of slots 50 and 56 which are both assigned to the first full-rate user, have the same sync-word S1. Preambles 72 and 78 of slots 52 and 58 which are both assigned to the second full-rate user, have the same sync-word S2. Preambles 74 and 80 of slots 54 and 60, each assigned to a different half-rate user, have different sync-word S3 and S6, respectively.
Accordingly, S1 is followed by S2, while S2 is followed by either S3 or S6.
Yow-Jong Liu, "Bi-Directional Equalization Technique for TDMA Communication systems over Land Mobile Radio Channels", GLOBECOM 1991 IEEE, p 1458-1462, describes a method for using the sync-word of the next slot which decodes the data of a slot according to its sync-word (forward analysis) and according to the sync-word of the next slot (backward analysis). When the sync-word of the next slot may have more than one option, then the data of the current slot is backward analyzed according to each of these options, thereby providing a plurality of data hypotheses. Finally, the most likely hypothesis is selected.
It will be appreciated that an analysis according to this method requires a considerable amount of computing resources and power, which can be critical for mobile communication devices having limited power resources.